Table 2.

Overview of key points of concern when evaluating the filtration performance of novel filter media and recommendations to address them

Points of Concern	Recommendations
Use of the term PMxx	• Do not use the term PMxx to define an aerosol fraction below a specific size without specifying sampling, measurement, and experimental protocols in detail. • Remember their meaning when using PM10 and PM2.5 terms and relate them to reference methods. • Use aerodynamic and physical size within the appropriate range. • Report the particle size distribution used for the experiment, indicating how it was measured.
Approach for measuring air filter efficiency	• Provide detailed information about the physical and chemical nature of the test aerosol. • The best methodology to understand and demonstrate fibrous filtration performance is measuring the efficiency as a function of the particle size. • Particle spectrometers can provide efficiency as a function of the particle size with a single test. • Photometers require challenging the filter with a monodisperse aerosol and measuring the reduction in particle concentration for one specific size. The procedure requires multiple tests as the investigated particle sizes. • If researchers are measuring a fixed polydisperse aerosol’s total particle number or mass, they must avoid expressing efficiency for specific PM fractions or at the MPPS. In this case, researchers can only compare filtration efficiency with benchmark media. • If possible, use a standardized test method following its qualification procedures.
Expressing air filter efficiency	• Air filtration efficiency as a function of particle size is necessary for calculating the removed PMxx fraction. • To calculate the air filter efficiency for a specific PMxx fraction, use the procedure explained in ISO 16890-1. • Expressing air filtration efficiency with a single number (e.g., 90%) requires a detailed test aerosol characterization to ensure meaningfulness and reproducibility.
Use of the photometer	• A photometer measurement using a polydisperse test aerosol cannot provide the efficiency corresponding to or below a specific particle size. • Single number efficiency obtained with a photometer is extremely sensitive to large particles in the tail of the particle size distribution. • Comparison among different filter media is valid only if the upstream distribution is identical.
Identification of MPPS and corresponding efficiency	• A photometer measurement using a polydisperse challenge aerosol cannot identify the MPPS. • Do not combine a photometer with a polydisperse aerosol having an MMD equal to the MPPS to obtain the maximum penetration. • When using a spectrometer, the upstream distribution must cover the particle size range where the MPPS falls. • Associate the efficiency at the MPPS with the media velocity because the MPPS is a function of the airflow rate.